The present invention relates to a ceramic, ceramic powder for scintillators for use in radioactive ray detectors for detecting X-ray, and a method for producing such ceramic and its powder.
Scintillators are materials emitting visible light upon receiving radioactive rays. Because the intensity of radioactive rays is in a proportional relation to the amount of light emitted from the scintillator, radioactive rays can be measured by a combination of the scintillator and a photodetector. Such technology is utilized mainly in medical apparatuses such as X-ray CTs, analysis apparatuses, non-destructive inspection apparatuses utilizing radioactive rays, apparatuses for detecting leaked radioactive rays, etc.
The scintillators are required to have properties such as high sensitivity to radioactive rays, high material uniformity and chemical stability. Further, when scintillators are used in apparatuses for rapidly detecting the changes of the intensity of radioactive rays such as X-ray CTs, it is important that they have a small attenuation time constant, which is defined as an elapsed time period until the luminescence intensity is attenuated to 1/e after the stop of irradiation, and a small afterglow, which is glow continuing after the stop of irradiation. Such scintillators are single crystals such as CdWO4, polycrystalline ceramics such as Gd2O2S:Pr,Ce,F, (Gd, Y)2O3:Eu,Pr, Gd3Ga5O12:Cr,Ce, etc.
Among these scintillators, the single crystal CdWO4 scintillator is disadvantageous in that it does not provide high luminescence intensity, that it cannot easily be worked because of cleavage, and that it contains a highly toxic ion of Cd. Though Gd2O2S:Pr,Ce,F has a high luminescence efficiency with small attenuation time constant and afterglow, it is disadvantageous in that it is produced through complicated processes, resulting in high production cost. Though (Gd, Y)2O3:Eu,Pr provides high luminescence intensity, it suffers from extremely large attenuation time constant. Also, Gd3Ga5O12:Cr,Ce is poor in luminescence intensity. The properties of these scintillators are shown in Table 1 below.
Accordingly, it has been desired to provide an inexpensive ceramic for a scintillator capable of providing high luminescence intensity with small attenuation time constant and afterglow.
Accordingly, an object of the present invention is to provide an inexpensive, oxide-type ceramic and its powder for a scintillator capable of providing high luminescence intensity with small attenuation time constant and afterglow.
Another object of the present invention is to provide a method for producing such an oxide-type ceramic and its powder for a scintillator at a low cost.
As a result of investigations on various oxide-type ceramics for scintillators in view of the above objects, the inventors have found that a garnet composition of (Gd, Ce)3(Al, Si, Ga)5O12 emits a high intensity of luminescence. This garnet composition can provide a sintered body having a cubic crystal structure with small optical anisotropy and high transmittancy.
Thus, the ceramic for a scintillator according to the present invention has a composition represented by the general formula:
Gd3-xCexAlySizGa5-y-zO12; 
wherein 0.001xe2x89xa6xxe2x89xa60.05, 1xe2x89xa6yxe2x89xa64, and 0.0015xe2x89xa6zxe2x89xa60.03.
The method for producing a ceramic powder having the above composition for a scintillator according to the present invention comprises the steps of mixing gadolinium oxide, aluminum oxide, gallium oxide, a cerium salt, a silicon compound and a fluorine compound in such proportions as to provide the above composition; and calcining the resultant mixture at a temperature of 1400-1600xc2x0 C.
The fluorine compound is preferably barium fluoride. Each of gadolinium oxide, aluminum oxide and gallium oxide preferably has an average diameter of 0.1-5 xcexcm.
The method for producing a sintered ceramic having the above composition for a scintillator comprises the steps of mixing gadolinium oxide, aluminum oxide, gallium oxide, a cerium salt, a silicon compound and a fluorine compound in such proportions as to provide the above composition; calcining the resultant mixture at a temperature of 1400-1600xc2x0 C.; disintegrating the resultant calcined body to ceramic powder; pressing the ceramic powder to provide a green body; and sintering the green body at a temperature of 1600-1700xc2x0 C. in a non-oxidizing atmosphere at 5xc3x97104 Pa or more.
The sintered ceramic may further be subjected to hot isostatic press sintering at a temperature of 1400-1600xc2x0 C. in an argon atmosphere.